Just make sure to always have a safety backup of your current FCP X application and of any older Libraries before updating to any new version, especially for Libraries with long and complex Projects. This way you can safely update while having the guarantee that if anything goes wrong you always have something to fall back upon.

A few days ago we dealt with a very specific problem someone had. He had updated a documentary that was started in 10.1.0, and after the update he had 5000 subtitles on the Project timeline that had lost their text. Ouch, indeed. Fortunately he had a backup of the 10.1.0 Library and of the 10.1.0 FCP X app. We solved the issue by exporting an Event XML of the old version and importing it in a fresh 10.1.3 Library. The underlying cause of the problem was that he had created the subtitles in a wrong format: